Crysis is an unrealistic game to benchmark most computers on. If that is your favorite game and one of the only ones you play then by all means, upgrade your computer to run it better. But for the most part you need a real super computer to be able to play Crysis smoothly.

The i7 offers a lot of great perks but I don't think it justifies the amount of $$$ you will be throwing into revamping your whole computer if you are running a Centrino processor. Most Centrinos would be a Core 2 Duo/Quad with DDR2/3 RAM.

The Pentium D on the other hand is a black sheep processor IMHO. I think you're better off getting a new processor, some RAM and maybe crossfiring that 3870 of yours with a twin. I think you got the right idea going and I would say go for it. If you want an extra kick I would recommend running a Velociraptor from Western Digital. It'll cut your load times in half, if not more.

If you want to get crazy, you can watercool your system. I've had a watercooled system for 6 years now and its a lot better than having your GPU fan going nuts.

Hope this helps, let me know if you have any questions.